The present invention relates to marine seismic energy sources adapted to be submerged in water aft of a seismic exploration vessel. More particularly, this invention relates to a streamlined, readily-towable marine seismic energy source for creating intense, swept-frequency and pulse-coded seismic signals in a body of water. The illustrative embodiment of the invention is a marine vibratory sound source having a sleek, fish-like configuration designed to be readily towed in the water with a minimum of drag.
There are prior vibratory-type marine seismic energy sources, but such equipment has been heavy, bulky, clumsy and awkward to deploy behind a seagoing vessel.
Certain prior hydraulically-actuated vibratory marine seismic energy sources were attempted to be mounted directly on the stern of a seagoing vessel. However, action/reaction forces generated by such stern-mounted, hydraulically-driven, vibratory, seismic energy equipment produced troublesome, disturbing and undesirable vibrations involving stern portions of the vessel itself. Such vibrations became severe in structural members, panels, braces and the like which happened to be vibrationally resonant with fundamental and/or harmonic (overtone) frequencies generated by a large, stern-mounted, powerful, hydraulically-driven vibratory seismic energy source.
Attempts to use prior vibratory seismic energy sources separated from the vessel itself and being towed through a body of water behind the vessel have experienced considerable difficulties. Their heavy weight and bulk have made them difficult to lift from shipboard over the stern and then lower into the water for deploying them. They similarly were difficult to retrieve from the water to return them onto shipboard.
Furthermore, large cross-sectional areas of prior marine seismic vibratory apparatus, i.e., their frontal areas, produced unacceptably large drag forces through water, thereby tugging backward against forward motion of towing vessels. Undesirable consequences of large drag forces are unduly large stresses in towing gear and wasteful large consumptions of fuel used in propelling towing vessels.
A sleek, low-drag-towable, vibrator marine seismic energy source for generating and propagating into a body of water intense, swept-frequency and pulse-coded sound energy signals.
Among numerous advantages provided by the illustrative embodiment of the invention are those resulting from its dramatically large vibration-coupling surface area in contact with the body of water. Yet, this large vibrational-coupling area is achieved while also designing a low-drag configuration.
The vibrator source has a streamlined towing head and a streamlined tail head mounted onto front and rear ends of a long cylindrical tubular wall. This wall is modular in construction comprising a plurality of cylinder sections forming cylinder chambers joined in end-to-end axial alignment.
Within these modular cylinder chambers is an axially vibratable multi-piston assembly having a long piston rod with multiple pistons mounted at axially-spaced positions on this long rod. One of these pistons is positioned in each of the cylinder chambers. These chambers have multiple ports opening out through the long cylindrical wall. An elongated circular cylindrical elastomeric bladder encircles the cylindrical wall forming a water-filled bladder chamber around the wall. Also, water fills the cylinder chambers and their ports. The nose of the streamlined towing head projects forward beyond the front end of the bladder, and the streamlined tail head projects aft beyond the aft end of the bladder.
An actuator piston in an actuator cylinder chamber is vibrated hydraulically by a remotely-controlled hydraulic circuit having a servo valve and manifold mounted within the streamlined towing head. Vibration of the actuator piston which is connected to the long piston rod of the multi-piston assembly serves to vibrate all of the pistons, thereby vibrating water from the cylinder chambers out and in through their ports into the water-filled bladder chamber for vibrating the exterior surface of the elastomeric bladder for propagating vibrator seismic energy signals into the surrounding body of water. These vibrator signals are swept-frequency signals and pulse-coded signals.
The bladder as shown has a diameter xe2x80x9cDxe2x80x9d of 18 inches and a length xe2x80x9cLxe2x80x9d of 118.5 inches for providing 6,700 square inches of vibrational-coupling area contacting the surrounding water.
These vibrator sources can be made smaller or larger in diameter, for example having an elastomeric bladder with a diameter in the range of about 6 inches to about 36 inches or more.
There are eight vibrator piston chambers shown. The vibrator""s modular construction facilitates making such vibrators with more than eight or less than eight vibrator piston chambers, as may be beneficial for usage in various marine seismic exploration and/or surveying applications.
A sensor responds to positioning of the multi-piston assembly in the vibrator for providing a signal to the shipboard controller indicating operating status of the multi-piston assembly for enabling synchronization of the present vibrator with companion vibrators being towed.
An axial passage in the long piston rod feeds low-pressure compressed air into the cylinder chambers for forming air cushions behind these pistons as they are vibrating water in their cylinder chambers. This low-pressure compressed air fills the streamlined towing head. Also, the axial passage extends within the long piston rod to its aft end and opens out into the interior of the tail head for filling the interior of the tail head with low-pressure compressed air.